VLBI SIGNAL PATH

The VLBI hardware at the VLA underwent major changes in 1993 and 1997. This section provides a brief description of this hardware. Four equipment racks are devoted exclusively to VLBI. Each is included in the sketch in Figure 6. These four racks are located in the VLBI area at the VLA.

Figure 6: VLBI equipment racks at the VLA.

Two ``tape racks'' contain the VLBA tape transports. These racks occupy the far left-hand side as one faces the four VLBI equipment racks. Each VLBA tape transport is a high speed longitudinal instrumentation tape recorder that uses 1 inch wide tape on reels 14 inches in diameter. The headstack contains 36 heads, 32 for data and 4 for system information, cross-track parity, or duplicate data if, for example, a head fails. The headstack can be moved under VME computer control transverse to the tape motion. The heads are much narrower than the spacing between heads, so multiple pass recording can be used with 14 passes, or more if not all heads are used in each pass. ``Thin'' instrumentation tapes can be used on either VLBA tape transport. A ``thin'' tape is $16~\mu$m thick and about 17600 feet long. Each VLBA tape transport can be used for recordings in VLBA or Mark 3 formats.

The next equipment rack to the right, called the ``miscellaneous rack'', houses a Phillips PM6680 counter, a 1 pps distributor, two Global Positioning System (GPS) receivers, a VME computer, an AC power switch, a 600 MHz VLBI upconverter, a data set, a power supply, four analog sum buffers, the VLBI T8 switch, and four square-law detectors to sample the T8 switch outputs. The Phillips counter counts the time difference between the Global Positioning System receiver and the 1 pps secure tick from the Master Station Clock. The WWV receiver should logically be in this rack, but due to space limitations this receiver is located in a rack, adjacent to the VLBI racks, used by the High Time Resolution Processor (see ``HTRP Operations Guide'', HTRP Memo No. 114, by M. M. McKinnon).

The VLBI T8 switch was installed in 1997 August. This switch is used to select the four VLA IFs that are to be passed to the 600 MHz VLBI upconverter to form the four VLBA IFs. The VLBI T8 switch can be configured under local (hand) control or under remote control by the VLA's Modcomp computers. In the latter case, the Modcomps establish whether the VLBI program involves the phased array or a single antenna; for the single-antenna case, the Modcomps also identify which specific antenna is being used. Recall from Section 13 that, for single-antenna VLBI, four T5 outputs, one for each of the antenna's VLA IFs, are sent to the VLBI T8 switch; while for phased-array VLBI, the T5 signals from all active antennas and all IFs are sent to the VLA correlator, where they are combined to yield an analog sum for each VLA IF. These four sums are sent to the analog sum buffers, where they are equalized and then passed to the VLBI T8 switch. The Modcomps configure the switch to route the appropriate quartet of VLA signals, one from each VLA IF, to the 600 MHz VLBI upconverter. For further diagnostic purposes, there are four square-law detectors that sample the upconverter inputs; the detector voltages can be read on the front panel of the upconverter.

The antenna being used for single-antenna VLBI is called the VLBI reference antenna. At any given time, three preferred VLBI antennas, one on each arm, will be available for use. If a given reference antenna develops problems during a single-antenna VLBI program, then the VLA operator can alter the VLBI subarray to select one of the other preferred VLBI antennas as the reference (see Section 16). If the VLBI T8 switch is running under remote control, it will automatically be reconfigured to reflect the antenna swap. Switching to a new preferred VLBI antenna on a given arm is a complicated process, since it involves changing cables at the D racks. Such changes must periodically be made based on operational considerations, such as the VLA's configuration.

Commencing in 2002 February, the auxilary inputs of the T8 switch have been connected to the ``donor'' antenna for VLA+PT programs. These A-configuration VLA programs add in the VLBA antenna at Pie Town (PT) by an optical fiber connection in order to gain an improvement in resolution. Since the VLA can only correlate 27 antennas, one antenna is designated the ``donor'' antenna to allow PT to be correlated. A natural use of the donor antenna is to give it to a concurrent VLBA program. The donor antenna is pre-selected at the beginning of each A configuration to be a good VLBI performer.

The 600 MHz VLBI upconverter is necessary because the VLBA IF distributors, located in the right-most VLBI rack (the ``data acquisition rack'' [DAR]), each require a signal between 500 MHz and 1000 MHz. In contrast, as just described in Section 13, the base band signals provided by the VLA are usually in the range from $0$ to $50$ MHz. In the VLBI upconverter, four inputs are mixed with a 600 MHz signal, with both upper and lower sidebands retained. This means that the four signals simultaneously provided by the upconverter are in the range 550 to 650 MHz.

Figure 7 shows the nominal signal path from the VLA IFs, to the VLBA IFs, and on to the DAR. This figure assumes the VLBI T8 switch is operated remotely and thereby automatically reconfigurable between VLBI programs using a single VLA antenna and those using the phased VLA. Note that all four VLA IFs are always available simultaneously and that both circular polarizations are always available simultaneously. In addition, there is a one-to-one mapping between the VLA IFs and VLBA IFs: VLA IF A $=$ VLBA IF A, VLA IF B $=$ VLBA IF B, VLA IF C $=$ VLBA IF C, and VLA IF D $=$ VLBA IF D.

Figure 7: Signal path into and out of the 600 MHz VLBI upconverter, and inside the DAR.

A VME site computer running VxWorks controls every aspect of the VLBA tape transport and the DAR. Control is based either on commands in the current VLBA control file, usually called ``bbbbbcrd.y'', where ``bbbbb'' is the program code (e.g., bq001); or on commands provided through the SCREEN program by knowledgeable operations staff. During a VLBI program, the VLA operator will use the SCREEN program to mount the VLBA tape(s), to monitor the BBCs and the formatter, and to check for errors on the formatter and tape recorder(s).

In addition to housing the VLBI IF distributers, the DAR contains several power supplies plus several modules housing the 5 MHz distribution, the samplers, the 32 MHz synthesizers, and eight BBC modules. These BBCs are called BBC1, BBC2, BBC3, BBC4, BBC5, BBC6, BBC7, and BBC8. Each BBC has outputs for both USB and LSB on the front of the module. There is a formatter clock within the DAR.

As illustrated in Figure 7, the four simultaneous signals passed from the 600 MHz VLBI upconverter to the DAR first encounter the VLBA IF distributors. There are two VLBA IF distributors, each with two channels, for a total of four channels. As already mentioned, these channels are named VLBA IF A, VLBA IF B, VLBA IF C, and VLBA IF D. Each VLBA IF distributor makes eight copies of each of its IF channels, one for each of the eight BBCs. At each BBC, the signal is brought to base band by mixing its input IF signal with a local oscillator whose frequency, set by the VME, is in the range 550 MHz to 650 MHz. Each BBC also does analog filtering, with filter selection under VME control. Each BBC can provide both the upper and lower sidebands as separate outputs, allowing for a total of 16 baseband (BB) channels. Allowed bandwidths per BBC are 0.0625, 0.125, 0.25, 0.5, 1, 2, 4, 8, and 16 MHz. Thus the 16 possible BB channels can in principal cover a recorded bandwidth up to 256 MHz. The BBC signals can be adjusted in amplitude, if so instructed by the VME. With automatic leveling turned on, the power in the output signals is kept nearly constant, which is important for the 2-bit (4-level) sampling possible for some VLBA-format recordings. These output signals are sent under VME control to the VLBA sampler and formatter for VLBA or Mark 3 programs.

Recall that $F_{VLA}$ is the frequency delivered by the VLA to DC at the input to the 600 MHz VLBI upconverter (see Section 13). Let firstlo(n)³ be the first LO frequency in MHz correpsonding to BB channel n, where n can range from 1 to 16. The NRAO program SCHED consults its frequency catalog to obtain a value for firstlo(n).

Then for observations at 0.33, 1.7, 5.0, 22, and 43 GHz, firstlo(n) can be obtained by subtracting 600 MHz from $F_{VLA}$ (see Equations 1, 2, 3, and 4):

$${\tt firstlo(n)} = F_{VLA} - 600.$$ (7)

For most VLBI programs at the VLA firstlo(n) will be identical for all n. Let bbsyn(n) be the VME-controlled synthesizer frequency setting in MHz of the BBC assigned to BB channel n. Then the sum of all the LOs in MHz for BB channel n will be given by

$${\tt losum(n)} = {\tt firstlo(n)} + {\tt bbsyn(n)}.$$ (8)

For observations at 8.4 and 15 GHz, firstlo(n) can be obtained by adding 600 MHz to $F_{VLA}$ (see Equations 5 and 6):

$${\tt firstlo(n)} = F_{VLA} + 600.$$ (9)

For most VLBI programs at the VLA firstlo(n) will be identical for all n. If bbsyn(n) is the VME-controlled synthesizer frequency setting in MHz of the BBC assigned to BB channel n, then the LO sum for BB channel n will be given by

$${\tt losum(n)} = {\tt firstlo(n)} - {\tt bbsyn(n)}.$$ (10)

Section 15 provides some complete examples of applying Equations 1 though 10 for the VLA.

For VLBA and Mark 3 programs, the VLBA samplers convert the analog BBC outputs to digital form. There are two samplers, each of which handles signals from 4 BBCs. Either 1-bit (2-level) or 2-bit (4-level) sampling may be selected. A single sample rate applies to all BB channels; rates available are 32, 16, 8, 4, 2, 1, or 0.5 Msamples per second on each channel. The VLBA formatter selects the desired bit streams from the samplers, adds timing and other information, fans the bit streams in or out (combines several slow input signals onto one tape track or spreads one fast input signal over several tape tracks), establishes the barrel roll, and sends the output signals to the tape recorders. As many as 32 bit streams can be formatted, with a bitstream:track multiplexing scheme of 4:1, 2:1, 1:1, 1:2, or 1:4, which allows for very flexible input signal to output tape track switching. As many as 32 (64) data tracks can be written to one (two) VLBA tape transport(s), with a record rate per track of 8, 4, or 2 Mbps. Up to 4 Mbits can be captured and sent to the VME and on to the AOC for various tests, including real time fringe checks.

No pulse calibration system is presently available at the VLA.

In the event of a power outage at the VLA, the VLBI equipment racks and the VLA master local oscillator systems will be maintained on the uninterruptible power supply for up to 15 m until either the emergency generators start or commercial power resumes.